(a) Field of the Invention
The present invention relates to a method and apparatus of codebook transformation in codebook-based multiple-input multiple-output (MIMO) precoding. More particularly, the present invention relates to a method and apparatus of codebook transformation for interference mitigation.
(b) Description of the Related Art
MIMO technology using multiple antennas is a communication scheme that improves channel capacity by generating a multiplexing gain or diversity gain from spatial freedom. Such MIMO technology can be divided into an open-loop scheme and a closed-loop scheme according to the use of a channel or channel-related information at the time of signal transmission.
The closed-loop scheme has a higher channel capacity than the open-loop scheme because it performs a precoding process using channel information for signal transmission. Also, it is necessary for a transmitter to acquire channel information between the transmitter and a receiver when transmitting a signal using the closed-loop MIMO technology, and there are various methods for acquiring channel information. For instance, there is a method in which a codebook representing a channel is defined, and a codeword having an optimum relationship with the channel measured at a receiver (e.g., the highest of intrinsic absolute values between a channel vector and a codeword vector or the highest Frobenius norm of a product of a channel matrix and a codeword matrix) is fed back to a transmitter.
A codeword, which is an element of the codebook, is indexed by a representative value of bits, and a value fed back to each transmitter is a bit value corresponding to a selected codeword. In order to use such a codebook-based precoding scheme, a codebook designed according to channel characteristics and a restriction on the number of feedback bits is required.
However, since the number of feedback bits is limited, it is not possible to design a codebook reflecting all of various channel characteristics. Accordingly, a method in which a codebook is designed for a channel with no correlation and the codebook is transformed for a highly correlated channel at long intervals was devised. This was devised using the fact that correlation characteristics of a channel do not change rapidly over time.
On the other hand, signal transmission using the closed-loop MIMO transmission shows superior performance to the open-loop MIMO transmission if no adjacent communication device where interference may occur exists. However, if an adjacent communication device where interference may occur exists, the closed-loop scheme may induce significant interference in the adjacent communication device, and the sum of transmission efficiencies including the transmission efficiency of the adjacent communication device is reduced, thus rendering the advantages of the closed-loop scheme meaningless.
Therefore, a closed-loop precoding scheme that takes into account interference in an adjacent communication device was proposed. Particularly, techniques such as precoding matrix index (PMI) restriction/recommendation/combination schemes for the codebook-based precoding scheme were proposed.
However, the proposed techniques are only applicable to a basic codebook to which codebook transformation is not applied. Thus, a channel where performance degradation occurs due to limitations of codebook design cannot eliminate interference in an adjacent communication device, or even if a codebook transformation technique is applied, its performance gain is not large. Subsequently, in order to overcome or make up for these problems, codebook transformation considering interference in an adjacent communication device is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.